The present invention relates in general to apparatus for use in emergency broadcast signal reception, and more particularly to an attention signal receiver for an emergency boradcast systems.
Generally, an emergency broadcast system includes a plurality of radio frequency transmitters originating at various locations. Some transmitters transmit amplitude modulated frequencies at standard broadcast wavelengths. Other transmitters transmit frequency modulated signals at standard broadcast wavelengths. The AM broadcast stations transmit the carrier amplitude modulated at 960 cycles and at 853 cycles. Similarly, FM broadcast stations transmit the carrier frequency modulated at 960 cycles and at 853 cycles.
Multiple scanning techniques are essential in the event one transmitter is idle. When such an event occurs, it is necessary to be able to receive the transmission from another or more distant transmitter. When an attention signal is detected, however, it is desirable to lock-in the first detected attention signal and to lock out the other attention signals.
The EBS System, Model 760, manufactured by Time and Frequency Technology, Inc. of Santa Clara, Calif., receives an emergency two-tone attention signal from an AM or FM station being monitored dependent on the type of receiver. The emergency two-tone signal is heard on an EBS System speaker after such a tone has been received for at least 8 seconds. The speaker is demuted after 8 seconds allowing the two-tone signal to be heard and a relay is energized to actuate an external alarm. The simultaneously generated two-tone signals are 853 H.sub.z and 960 H.sub.z. In another version of the EBS System, the detection of 1 KH.sub.z tone for a period of time greater than 10 seconds through a ten second time delay circuit operates an alarm. An audio squelch circuit in a two-tone encoder/decoder operates to energize or excite an alarm. The EBS System, Model 760, can be either an AM Receiver or an FM Receiver. A timing circuit of a ten second time delay is a signal averaging integrator which eliminates false turn-on by noise signals.
An EBS decoder manufactured by Audio Services, Inc. of Detroit, Mich. and other manufacturers detects two-tone EBS alert signals and can be used with standard AM or standard FM broadcast receivers. Upon receipt of two simultaneously transmitted tones via the receiver, the EBS decoder monitors the signal silently for ten seconds to guard against false triggering. It then automatically demutes the speaker for aural monitoring and an external alarm is activated. The two-tone alert signals are 853 H.sub.z and 960 H.sub.z.
In the patent to Myers et al., No. 3,470,481, issued on Sept. 30, 1979, for Multichannel Communication Receiver With Automatic Sampling And Lock In One Channel, there is disclosed a receiver in which several incoming frequencies from differently located transmitters are alternately detected. A plurality of local oscillators of different frequencies are alternately excited for sequential scanning of incoming carrier frequencies by a sequential scanning circuit. As soon as an attention signal is detected from one of the transmitters, a disabling signal is applied to the sequential scanning circuit for locking the receiver to the transmission from the transmitter whose attention signal was first detected.
In the U.S. Patent to Gleason, No. 3,482,166, issued on Dec. 2, 1969, for Multi-Frequency Receiver With Automatic Monitoring Of Channels With One Channel Priority, there is disclosed a superheterodyne receiver for receiving the transmission from a plurality of transmitters. A plurality of local oscillators are alternately operated for mixing with incoming carrier frequencies. A squelch circuit mutes the audio output until an attention signal is received. After the attention signal is detected, the squelch circuit demutes the audio output and disables the oscillator switching circuit to lock the receiver to the channel of the transmitter of the attention signal.
The U.S. Patent to Zerega, Jr., No. 3,623,106, issued on Nov. 23, 1971, for Multifrequency Receiver Employing Tone-Coded Squelch With Automatic Channel Selection discloses a multichannel receiver in which a plurality of local oscillators are alternately excited by a sequencing circuit. When an attention signal is detected, a fast squelching circuit of a relatively short time delay disables the sequencing circuit. This locks or latches the receiver to the carrier detected with the attention signal. If the latched carrier is of sufficient duration for a tone operated squelching circuit to operate, then the tone operated squelching circuit will disable the sequencing circuit. The time delay for the time operated squelching circuit is relatively long.
The U.S. Patent to Giles, Jr. et. al., No. 3,169,788, issued on Nov. 9, 1971, for a Circuit For Giving Priority To One Of A Plurality Of Automatically Monitored Channels In A Receiver discloses a superheteodyne receiver in which a plurality of oscillators are sequentially operated by a sequential scanning switch. When a signal is received, a noise squelch disables the sequential scanning switch to lock the receiver to the channel in which the signal is present.
As for the U.S. Patent to Tomsa et al., No. 3,617,895, issued on Nov. 2, 1971, for Multifrequency Receiver With Automatic Channel Selection And Priority Channel Monitoring, it discloses a receiver with a plurality of local oscillators operated in sequence by a switching circuit. When a signal is detected, a squelching circuit latches the switching system so that the receiver is held to the carrier having the signal.
The U.S. Patent to Bryant, No. 4,103,235, issued on July 25, 1978, for Two-Tone Attention Signal Broadcasting System discloses a receiver-decoder for emergency broadcast systems. The decoder receives an audio signal and filters out the two tones of an attention signal. When the two tone attention signal is received, the speaker of the receiver is demuted for an announcement. A timer is provided to reset the decoder after a predetermined time interval.